Light-emitting semiconductor device having a quantum well active layer, and method of fabrication

ABSTRACT

A light-emitting diode has a low-resistivity silicon substrate on which there are laminated a buffer layer, an n-type lower confining layer, an active layer of multiple quantum well configuration, and a p-type upper confining layer. The active layer is constituted of cyclic alternations of a barrier sublayer of InGaN, a first complementary sublayer of AlGaInN, a well sublayer of InGaN, and a second complementary sublayer of AlGaInN. The proportions of the noted ingredients of the active sublayers are all specified. The first and the second complementary sublayers prevent the evaporation or diffusion of indium from the neighboring sublayers.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of Application PCT/JP2003/013858, filed Oct. 29,2003, which claims priority to Japanese Patent Application No.2002-322826 filed Nov. 6, 2002.

BACKGROUND OF THE INVENTION

This invention relates to light-emitting semiconductor devices such aswhat is called the light-emitting diode in common parlance, particularlyto those employing nitrides or nitride-based compounds assemiconductors, and more particularly to an active layer of improvedquantum well structure included in such light-emitting devices. Theinvention also pertains to a method of making such light-emittingsemiconductor devices.

Japanese Unexamined Patent Publication Nos. 8-264832 and 2001-313421 arehereby cited as dealing with high-intensity light-emitting nitridesemiconductor devices. For production of light ranging from ultravioletto green, these known devices have their active region made from theclass of gallium nitrides that are generally defined asIn_(x)Al_(y)Ga_(1-x-y)N where x and y are both equal to or greater thanzero and equal to or less than one and where the sum of x and y is equalto or greater than zero and equal to or less than one.

As heretofore constructed, light-emitting semiconductor devices have asubstrate of sapphire, silicon carbide, or silicon on which there aregrown gallium nitride semiconductor layers. In the case of a sapphiresubstrate, for instance, there is formed thereon a lamination of GaNbuffer layers, a silicon-doped n-type GaN contact layer, an n-type AlGaNsemiconductor layer, an active region of quantum well structure, ap-type AlGaN semiconductor layer, and a magnesium-doped p-type GaNcontact layer, in that order from immediately over the substrate fartheraway therefrom.

Two different types of quantum well structures are known in the art:multiquantum well and single quantum well. The active region of bothtypes includes a well layer or layers and barrier layers. Both welllayers and barrier layers are made from InGaN for example, but the welllayers contain indium in a greater proportion than do the barrierlayers. The well layers are usually grown at temperatures not exceeding800° C. with a view both to prevention of the decomposition of indiumand to higher crystallinity. The usual practice in the art has been togrow the barrier layers approximately at the same temperature as thewell layers. The resulting barrier layers have not necessarily beensatisfactory in crystallinity, bringing about some difficulties andinconveniences set forth hereinbelow.

The barrier layers of poor crystallinity are, first of all, incapable ofsufficiently restricting the vaporization of indium from the welllayers, thereby allowing the well layers to deteriorate incrystallinity, too. Another inconvenience is the mutual diffusions ofindium from the well layers and gallium from the barrier layers, beyondthe interfaces between the well and barrier layers. The well layers havethus been easy to become irregular in virtual thickness, resulting influctuations in the wavelength of the light emitted. Furthermore, duringthe subsequent growth of the noted p-type GaN layer on the active regionat 1100° C. or so, the barrier layers of poor crystallinity have furtherpromoted both of indium evaporation from the well layers and the mutualdiffusions of indium and gallium beyond the interfaces between the welland barrier layers. The diffusion of impurities such as magnesium fromp-type GaN layer to active region, with the consequent deterioration ofthe crystallinity of the active region, has also had to be feared.

SUMMARY OF THE INVENTION

The present invention aims at provision of a light-emittingsemiconductor device having a quantum-well active layer which is freefrom the difficulties and inconveniences of the prior art discussed inthe foregoing, and of a method of fabricating such a quantum-welllight-emitting device.

Briefly, the method of this invention is specifically directed to howthe quantum-well active layer is fabricated in a light-emittingsemiconductor device of the kind defined. The method dictates, first ofall, the creation of a barrier sublayer of the active layer byorganometallic chemical vapor deposition (OMCVD) on a preexistingsemiconductor layer of a prescribed conductivity type included in thelight-emitting semiconductor device. The barrier sublayer is ofIn_(y)Ga_(1-y)N where the subscript y is a numeral that is greater thanzero and equal to or less than 0.5. Then a first complementary sublayeris formed by OMCVD on the barrier sublayer, the first complementarysublayer being of Al_(a)Ga_(b)In_(1-a-b)N where the subscript a is anumeral that is greater than zero; the subscript b is a numeral that isless than one; and the sum of a and b is equal to or less than one. Thena well sublayer is formed by OMCVD on the first complementary sublayer,the well sublayer being of In_(x)Ga_(1-x)N where the subscript x is anumeral that is greater than zero, equal to or less than 0.5, andgreater than y in the formula above defining materials for the barriersublayer. Then a second complementary sublayer is formed by OMCVD on thewell sublayer, the second complementary sublayer being ofAl_(a′)Ga_(b′)In_(1-a′-b′)N where the subscript a′ is a numeral that isgreater than zero; the subscript b′ is a numeral that is less than one;and the sum of a′ and b′ is equal to or less than one. Then anotherbarrier sublayer is formed by OMCVD on the second complementarysublayer, the second recited barrier sublayer being of In_(y)Ga_(1-y)Nwhere the subscript y is a numeral that is greater than zero and equalto or less than 0.5.

Another aspect of the invention concerns the light-emittingsemiconductor device having an active layer fabricated by the abovesummarized method. The active layer is a lamination of the barriersublayer, the first complementary sublayer, the well sublayer, thesecond complementary sublayer, and the other barrier sublayer. Thecompositions of all these sublayers are as set forth above.

Made from an aluminum-nitride-based semiconductor, the secondcomplementary sublayer functions to prevent the evaporation of indiumand other ingredients from the well sublayer, and the mutual diffusionsof indium from the well sublayer and gallium from the barrier sublayer.The first complementary sublayer of similar, but not necessarilyidentical, composition also serves the prevent the mutual diffusions ofindium from the well sublayer and gallium from the barrier sublayer. Theresult is a significant improvement in the crystallinity of the activelayer.

It is preferred that the active layer be of multiquantum wellconfiguration, as in the preferred embodiments of the invention to bepresented subsequently. A multiquantum well active layer is producibleby cyclically repeating the sequential fabrication of one barriersublayer, one first complementary sublayer, one well sublayer, and onesecond complementary sublayer a required number of times, and, finally,by fabricating an additional barrier layer on the last of such sets ofsublayers.

In order to prevent indium evaporation and diffusion from the well layeror layers while the active layer as a whole is held relatively low inresistivity, the second complementary sublayer or sublayers may be madegreater in aluminum content than the first complementary sublayer orsublayers. Generally, the higher the aluminum proportions a and a′ ofthe Al_(a)Ga_(b)In_(1-a-b)N first complementary sublayer or sublayersand Al_(a′)Ga_(b′)In_(1-a′-b′)N second complementary sublayer orsublayers, the better will they reduce indium evaporation and diffusion.However, the first and second complementary sublayers will become moreresistive as they grow higher in aluminum contents. As a compromise, andparticularly in cases where the prevention of indium evaporation is moreimportant, the second complementary sublayer or sublayers should be madeas aforesaid greater in aluminum content than the first complementarysublayer or sublayers. This is believed to be the most practicalsolution to the problems of indium evaporation and indium and galliumdiffusion.

Also, preferably, the second complementary sublayer or sublayers may bemade thicker than the first complementary sublayer or sublayers. Thisdifference in thickness will yield results similar to those obtainedwhen the second complementary sublayer or sublayers are made greater inaluminum content as above than the first complementary sublayer orsublayers.

As has been set forth in the foregoing summary of the invention, theactive layer is formed on a preexisting semiconductor layer of aprescribed conductivity type included in the light-emittingsemiconductor device. This preexisting semiconductor layer is a lowerconfining layer, which is formed on a doped silicon substrate via abuffer layer, in one embodiment of the invention, and the buffer layeritself in another.

In either case the buffer layer may preferably take the form of alamination of alternating first and second sublayers, each firstsublayer being of Al_(x)Ga_(1-x)N where the subscript x is a numeralthat is greater than zero and equal to or less than one and having athickness determined to offer a quantum-mechanical tunnel effect, eachsecond sublayer being of Al_(y)Ga_(1-y)N where the subscript y is equalto or greater than zero, less than one, and less than x in the formuladefining materials for the first sublayer, and having a thickness in therange of from about 10 to about 500 nanometers. The buffer layer willthen offer minimal electrical resistance.

The above and other objects, features and advantages of this inventionwill become more apparent, and the invention itself will best beunderstood, from a study of the following description and appendedclaims, with reference had to the attached drawings showing thepreferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section through a first preferred form oflight-emitting diode embodying the principles of this invention.

FIG. 2 is a greatly enlarged, partial section through the buffer layerof the light-emitting diode of FIG. 1.

FIG. 3 is a greatly enlarged, partial section through the active layerof the light-emitting diode of FIG. 1.

FIG. 4 is a schematic cross section through another preferred form oflight-emitting diode according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The gallium nitride light-emitting device according to the inventionwill now be described more specifically in terms of the light-emittingdiode (LED) shown in FIG. 1. The method of fabricating this LED will bepresented following the detailed explanation of its construction.

The illustrated LED has a low-resistivity substrate 1 of impurity-dopedmonocrystalline silicon. Formed on this substrate 1 is, first of all, ann-type buffer layer 2 shown in detail in FIG. 2, to which reference willbe had presently. The buffer layer 2 is overlain by three mainsemiconductor layers 3, 4 and 5, which are arranged one on top ofanother in that order from the lowermost one upward. The lowermost mainsemiconductor layer 3 is an n-type cladding or confining layer; theintermediate semiconductor layer 4 is an active layer; and the topmostmain semiconductor layer 5 is a p-type cladding or confining layer. Thebuffer layer 2 might be considered a part of the n-type semiconductorlayer. Additional components of the LED are an anode or front electrode6 _(a) on top of the LED chip or of the p-type semiconductor layer 5,and a cathode or back electrode 6 _(b) on the back of the LED chip or ofthe substrate 1.

Preferably, the substrate 1 is of n-type monocrystalline silicon dopedwith arsenic as a conductivity type determinant. The major surface 1_(a) of this silicon substrate 1 on which is formed the buffer layer 2is exactly (111) in terms of Miller indices. The impurity concentrationof the silicon substrate 1 is in the range of from 5×10¹⁸ to 5×10¹⁹cm⁻³, and its resistivity in the range of from 0.0001 to 0.0100ohm-centimeter. The silicon substrate 1 in this resistivity range iselectrically conductive enough to provide a current path between anode 6_(a) and cathode 6 _(b) in the operation of the LED. The siliconsubstrate 1 should be sufficiently thick (e.g. 350 micrometers) to serveas a mechanical support for the buffer layer 2 and main semiconductorlayers 3, 4 and 5 formed thereon.

As drawn fragmentarily and on a greatly enlarged scale in FIG. 2, then-type buffer layer 2 is a lamination of a first and a second nitridesemiconductor sublayer 2 _(a) and 2 _(b) arranged alternately a requirednumber of times. Fifty first sublayers 2 _(a) and fifty second sublayers2 _(b) alternate in this particular embodiment of the invention.

The first buffer sublayers 2 _(a) are made from any of the nitrides thatare generally expressed by the formula:Al_(x)Ga_(1-x)Nwhere the subscript x is a numeral that is greater than zero and equalto or less than one. Specific examples meeting this formula are aluminumnitride (AlN) and aluminum gallium nitride (AlGaN), out of which AlN (xis one in the formula above) is employed in this particular embodiment.The first buffer sublayers 2 _(a) are electrically insulating andsufficiently thin to offer the quantum-mechanical tunnel effect. Thefirst buffer sublayers 2 _(a) are, moreover, closer in both latticeconstant and thermal expansion coefficient to the silicon substrate 1,and therefore higher in buffering capability, than are the second buffersublayers 2 _(b).

The second buffer sublayers 2 _(b) are made from, in addition to ann-type determinant, any of the nitrides that are generally expressed bythe formula:Al_(y)Ga_(1-y)Nwhere the subscript y is a numeral that is equal to or greater than zeroand less than one and, additionally less than x in the formula abovedefining the materials for the first buffer sublayers 2 _(a). Thus thesecond buffer sublayers 2 _(b) can be made from GaN or AlGaN plus ann-type determinant. The second buffer sublayers 2 _(b) are each greaterin thickness than each first buffer sublayer 2 _(a). Preferably, incases where n-type AlGaN is employed for the second buffer sublayers 2_(b), the value of y should be greater than zero and less than 0.8 inorder to keep the second buffer sublayers from becoming too high inelectrical resistivity. The second buffer sublayers 2 _(b) function asconductors or semiconductors for electrical connection of the firstbuffer sublayers 2 _(a).

The first buffer sublayers 2 _(a) are each from 0.5 to 10 nanometersthick, preferably from one to eight nanometers. The three overlying mainsemiconductor layers 3–5 would not all gain a desired degree of flatnessif the first buffer sublayers 2 _(a) were each less than 0.5 nanometerthick. If they exceed 10 nanometers in thickness, on the other hand, thequantum-mechanical tunnel effect would not be obtained, and the bufferlayer 2 as a whole would inconveniently rise in electrical resistivity.

The second buffer sublayers 2 _(b) are each from 10 to 500 nanometersthick, preferably from 10 to 300 nanometers. Should each second buffersublayer 2 _(b) be less than 10 nanometers thick, the energy bandbetween substrate 1 and the second buffer sublayer 2 _(b) would becomeso discontinuous that both resistivity and voltage would becomeunnecessarily high between anode 6 _(a) and cathode 6 _(b) during theoperation of the device. Also, if each second buffer sublayer 2 _(b)were less than 10 nanometers thick, electrical connection would not beestablished sufficiently between the two adjoining first buffersublayers 2 _(a) on the opposite sides of each second buffer sublayer 2_(b), resulting in an increase in the resistivity of the buffer layer 2.Should each second buffer sublayer 2 _(b) exceed 500 nanometers inthickness, on the other hand, then the first buffer sublayers 2 _(a)would become so small in size in relation to the complete buffer layer 2that the latter would not perform its intended functions to the full.The intended functions of the buffer layer 2 are to improve thecrystallinity and flatness of the overlying main semiconductor layers3–5.

Overlying the buffer layer 2 of the FIG. 2 configuration, the n-typemain semiconductor layer 3 is made from any of the n-type galliumnitrides semiconductors that are generally expressed as:Al_(x)Ga_(1-x)Nwhere the subscript x is a numeral that is equal to or greater than zeroand less than one. The particular semiconductors meeting theserequirements are n-type GaN and n-type AlGaN. The particular materialemployed in this embodiment of the invention is n-type GaN (x is zero inthe formula above).

As illustrated in detail in FIG. 3, the multi-quantum-well active layer4 on the n-type main semiconductor layer 3 is a lamination of barriersublayers 7, well sublayers 8, first complementary sublayers 9, andsecond complementary sublayers 10. It will be observed from this figure,taken together with FIG. 1, that the active layer 4 has a whole has onemajor surface 4 _(a), which is contiguous to the n-type mainsemiconductor layer 3, and another major surface 4 _(b) on which isformed the p-type main semiconductor layer 5. One barrier sublayer 7,one first complementary sublayer 9, one well sublayer 8, and one secondcomplementary sublayer 10, arranged in that order from the one majorsurface 4 _(a) of the active layer 4 toward the other 4 _(b), constituteone of a required number of sets of such sublayers of the active layer.An additional barrier sublayer 7 overlies the topmost set of activesublayers 7–10. This arrangement of the active sublayers may be restatedthat the required number of sublayer sets, each consisting of one firstcomplementary sublayer 9, one well sublayer 8, one second complementarysublayer 10, and one barrier sublayer 7, are laminated upon thebottommost barrier sublayer 7 directly overlying the n-type mainsemiconductor layer 3. The active layer 4 is composed of fifteen wellsublayers 8, fifteen first complementary sublayers 9, fifteen secondcomplementary sublayers 10 ⁻, and sixteen barrier sublayers 7 in thisembodiment of the invention.

The barrier sublayers 7 of the active layer 4 are fabricated from any ofthe indium gallium nitride semiconductors that can be generally definedas:In_(y)Ga_(1-y)Nwhere the subscript y is a numeral that is greater than zero and equalto or less than 0.5. A preferred range of values for the subscript y isfrom 0.01 to 0.10, and most desirably from 0.01 to 0.05. The barriersublayers 7 are made from In_(0.03)Ga_(0.97) (y is 0.03 in the formulaabove) in this particular embodiment of the invention. Each barriersublayer 7 can be from five to 20 nanometers thick and is 10 nanometersin this embodiment. The barrier sublayers 7 have a bandgap in the energyband diagram that is greater than that of the well sublayers 8.

The well sublayers 8 of the active layer 4 are made from any of theindium gallium nitride semiconductors that can be generally defined as:In_(x)Ga_(1-x)Nwhere the subscript x is a numeral that is greater than zero, equal toor less than 0.5, and greater than the subscript y of the formula abovedefining the materials for the barrier sublayers 7. The indiumproportion x of the well sublayer materials is made higher than that yof barrier sublayer materials in order for the well sublayers to be lessin bandgap than the barrier sublayers. More specifically, x is in therange of from 0.01 to 0.40, preferably from 0.02 to 0.30. The particularmaterial employed for the well sublayers 8 in this embodiment of theinvention is In_(0.2)Ga_(0.8)N (x is 0.2). Each well sublayer 8 may befrom one to ten nanometers thick and is three nanometers in thisembodiment.

Each interposed between one well sublayer 8 and one underlying barriersublayer 7, the first complementary sublayers 9 of the active layer 4are intended for preventing the mutual diffusion of gallium from thebarrier sublayers and of indium from the well sublayers. These firstcomplementary sublayers 9 are made from any of the aluminum-containingnitride semiconductors that are generally expressed by the formula:Al_(a)Ga_(b)In_(1-a-b)Nwhere the subscript a is a numeral that is greater than zero; thesubscript b is a numeral that is less than one; and the sum of a and bis equal to or less than one.

The specific substances that meet these requirements are AlN, AlGaN, andAlGaInN. The aluminum proportion a is preferably in the range of from0.01 to 0.90, most desirably from 0.1 to 0.7. The particular substanceemployed for the first complementary sublayers 9 is Al_(0.5)Ga_(0.5)N(both a and b are 0.5). The aluminum proportion a for the firstcomplementary sublayers 9 may be made less than the aluminum proportiona′ for the second complementary sublayers 10, which are to be detailedsubsequently, for reasons that are also to be set forth in connectionwith the second complementary sublayers. Each first complementarysublayer 9 is from 0.1 to 20 nanometers thick, preferably from 0.5 to1.5 nanometers thick, and 0.8 nanometer thick in this particularembodiment. The first complementary sublayers 9 may be made thinner thanthe second complementary sublayers 10 for the same reasons the aluminumproportion a is made less than the aluminum proportion a′ as above.

The second complementary sublayers 10 of the active layer 4, eachinterposed between one well sublayer 8 and one overlying barriersublayer 7, serve mainly to prevent evaporation of indium from the wellsublayers. These second complementary sublayers 10 are made from any ofthe aluminum-containing nitride semiconductors that are generallyexpressed by the formula:Al_(a′)Ga_(b′)In_(1-a′-b′)Nwhere the subscript a′ is a numeral that is greater than zero and,preferably, than the subscript a in the formula above defining thematerials for the first complementary sublayers 9 of the active layer 4;the subscript b′ is a numeral that is equal to or less than one; and thesum of a′ and b′ is equal to or less than one.

The specific substances that may be employed for the secondcomplementary sublayers 10 are AlN, AlGaN, and AlGaInN. Preferably, thealuminum proportion a′ in the formula for the second complementarysublayers 10 is in the range of from 0.01 to 1.00, and from 0.1 to 1.0for the best results. The aluminum proportion a′ is one, and the galliumproportion b′ is zero, in this particular embodiment; in other words,the second complementary sublayers 10 are made from AlN.

Furthermore, in order effectively to prevent the vaporization of indiumby the second complementary sublayers 10, the aluminum proportion a′ ofthese sublayers should be greater than the aluminum proportion a of thefirst sublayers 9, and the second complementary sublayers should bethicker than the first complementary sublayers. Each secondcomplementary sublayer 10 is from 0.5 to 3.0 nanometers thick,preferably from one to two nanometers thick, and 1.5 nanometers thick inthis particular embodiment.

As is clear from the foregoing, the first complementary sublayers 9 ofthe active layer 4 are intended primarily for prevention of the mutualdiffusion of gallium from the barrier sublayers 7 and of indium from thewell sublayers 8. The second complementary sublayers 10 on the otherhand are intended primarily for prevention of the vaporization of indiumfrom the well sublayers 8. The aluminum contents of both first andsecond complementary sublayers 9 and 10 are effective for the preventionof both the mutual diffusions of gallium and indium and the vaporizationof indium.

However, the greater the aluminum proportions a and a′ are in thecomplementary sublayers 9 and 10, the higher will these sublayers be inelectrical resistivity. The complementary sublayers 9 and 10 will alsogain in electrical resistivity as they grow thicker. Limitations musttherefore be imposed upon both the aluminum proportions a and a′ andthicknesses of the complementary sublayers 9 and 10. Improvement in theperformance of the active layer 4 due to the prevention of indiumvaporization by the aluminum content of the second complementarysublayers 10 is more pronounced than that due to the prevention of themutual diffusions of gallium and indium by the aluminum content of thefirst complementary sublayers 9. It is therefore practical to make thealuminum proportion a′ of the second complementary sublayers 10 higherthan the aluminum proportion a of the first complementary sublayers 9,and to make the second complementary sublayers 10 thicker than the firstcomplementary sublayers 9.

With reference back to FIG. 1 the p-type main semiconductor layer 5 onthe active layer 4 of foregoing configuration is fabricated from, inaddition to a p-type dopant, either GaN or AlGaN which can be generallyexpressed as:Al_(y)Ga_(1-y)Nwhere the subscript y is a numeral that is equal to or greater than zeroand less than one. The particular substance employed in this embodimentis p-type GaN (y is zero in the formula above). Optionally, anadditional gallium nitride layer such as that of p-type AlGaN might beinterposed between active layer 4 and p-type main semiconductor layer 5.

Mounted centrally on the surface of the p-type main semiconductor layer5, the anode 6 _(a) is electrically connected to that layer 5. The anode6a could be mounted to the main semiconductor layer 5 via a p-typecontact layer. Another possible modification is the interposition of aknown current limiting layer between active layer 4 and anode 6 _(a).The cathode 6 _(b) is mounted to the underside of the substrate 1 andelectrically coupled thereto.

What follows is the disclosure of a preferred method of making the LEDof the FIGS. 1–3 construction. The method started with the preparationof the substrate 1 which was of silicon to which there was added ann-type dopant to provide an n⁺-type silicon substrate. The major surface1 _(a) of the silicon substrate 1, on which was to be formed the bufferlayer 2, was precisely (111) in terms of Miller indices, althoughdeviations from this plane to the extent of several angular degrees arepermissible.

Then the buffer layer 2 was formed on the major surface 1 _(a) of thesubstrate 1 as part of the first conductivity type semiconductor region.As depicted in detail in FIG. 2, the buffer layer 2 was a lamination ofalternating AlN sublayers 2 _(a) and GaN sublayers 2 _(b), both made byknown organometallic chemical vapor deposition (OMCVD), known also asmetalorganic chemical vapor deposition (MOCVD).

Introduced into the reactor of a commercially available OMCVD system,the monocrystalline silicon substrate 1 was first put to thermalannealing for stripping the oxide films off its surfaces. Thentrimethylaluminum (TMA) gas and ammonia (NH₃) gas was charged into thereactor for 27 seconds, thereby growing a first buffer sublayer 2 _(a)of AlN to a thickness of approximately five nanometers on the surface 1_(a) of the substrate 1. The TMA gas (i.e. aluminum) was charged at arate of approximately 63 micromoles per minute, and the NH₃ gas at arate of approximately 0.14 mole per minute, with the substrate 1 heatedto a temperature of 1120° C.

Then, with the supply of TMA gas terminated and the substratetemperature held at 1120° C., trimethylgallium (TMG) gas, NH₃ gas andsilane (SiH₄) gas were introduced into the reactor for 83 seconds. Therewas thus grown on the first buffer sublayer 2 _(a) a second buffersublayer 2 _(b) of n-type GaN to a thickness of approximately 30nanometers. The SiH₄ gas was intended for addition of silicon into theGaN sublayer 2 _(b) as an n-type dopant. The TMG gas (i.e. gallium) wascharged at a rate of 63 micromoles per minute, the NH₃ gas at a rate of0.14 mole per minute, and the SiH₄ gas (i.e. silicon) at a rate of 21nanomoles per minute.

The above described process of fabricating one first AlN buffer sublayer2 _(a) and one second GaN buffer sublayer 2 _(b) was repeated 50 times.There was thus completed the buffer layer 2 in the form of a laminationof 100 alternating first AlN sublayers 2 _(a) and second GaN sublayers 2_(b). It is of course understood that the buffer layer 2 can be of anarbitrary number (i.e. 25) of alternating sublayers 2 _(a) and 2 _(b).

Next came the steps of growing the n-type main semiconductor layer 3,active layer 4, and p-type main semiconductor layer 5 on the bufferlayer 2 by OMCVD. The n-type main semiconductor layer 3 of n-type GaNwas first formed on the buffer layer 2 by introducing TMG gas, NH₃ gasand SiH₄ gas into the OMCVD reactor, with the substrate 1, together withthe buffer layer 2 thereon, held at a temperature of 1040° C. The TMGgas was introduced at a rate of 4.3 micromoles per minute, the NH₃ gasat a rate of 53.6 millimoles per minute, and the SiH₄ gas at a rate of1.5 nanomoles per minute. The SiH₄ gas was intended for introduction ofsilicon into the n-type main semiconductor layer 3 as an n-typeimpurity.

The n-type main semiconductor layer 3 was thus grown to a thickness of0.2 micrometer, which is significantly less than the usual thickness(from 4.0 to 5.0 micrometer) of the n-type semiconductor layers of priorart LEDs of comparable design. The impurity concentration of this mainsemiconductor layer 3 was approximately 3×10¹⁸ cm⁻³, sufficiently lessthan that of the substrate 1. Overlying the substrate 1 via the bufferlayer 2, instead of directly, the main semiconductor layer 3 could begrown at as high a temperature as 1040° C.

The next step was the creation of the active layer 4 which, as shown indetail in FIG. 3, is a lamination of alternating barrier sublayers 7,well sublayers 8, and first and second complementary sublayers 9 and 10.The lowermost barrier sublayer 7 of In_(y)Ga_(1-y)N was first formed onthe main semiconductor layer 3 by OMCVD. The substrate 1 with the bufferlayer 2 and main semiconductor layer 3 thereon was heated to 800° C.,and the gases of TMG, NH₃, and trimethyl indium (TMI) were charged intothe OMCVD reactor. The TMG gas was charged at a rate of 1.1 micromolesper minute, and the TMI gas at 1.0 micromole per minute. The barriersublayer 7 was grown to a thickness of approximately 10 nanometers, andits composition was In_(0.03)Ga_(0.97)N.

Then one first complementary sublayer 9 of Al_(a)Ga_(b)In_(1-a-b)N wasgrown on the lowermost barrier sublayer 7. The gases of TMG, NH₃, andtrimethylaluminum (TMA) were charged into the OMCVD reactor, the TMG gasat a rate of 1.1 micromoles per minute, and the TMA gas at a rate of 1.2micromoles per minute. The composition of the resulting firstcomplementary sublayer 9 was Al_(0.5)Ga_(0.5)N (a=0.5, b=0.5).

Then one well sublayer 8 of In_(x)Ga_(1-x)N was grown on the firstcomplementary sublayer 9. The gases of TMG, NH₃, and TMI were chargedinto the OMCVD reactor, the TMG gas at a rate of 1.1 micromoles perminute, and the TMI gas at a rate of 4.5 micromoles per minute. The wellsublayer 8 was grown to a thickness of approximately three nanometers.

Then one second complementary sublayer 10 of Al_(a′)Ga_(b′)In_(1-a′-b′)Nwas grown on the well sublayer 8 by introducing both NH₃ and TMA gasesinto the OMCVD reactor. The TMA gas was introduced at a rate of 2.4micromoles per minute. The composition of the resulting secondcomplementary sublayer 10 was AlN (a′=1, b′=0).

The foregoing processes of fabricating one barrier sublayer 7, one firstcomplementary sublayer 9, one well sublayer 8 and one secondcomplementary sublayer 10 were cyclically repeated until the activelayer 4 was completed which consisted of the required alternations ofthese sublayers.

The next step was the fabrication of the p-type main semiconductor layer5, FIG. 1, on the active layer 4 which had been completed as above. Thesubstrate 1 with the various layers and sublayers so far made thereonwas heated to 1040° C., and there were introduced into the OMCVD reactorthe gases of TMG, NH₃, and bis-cyclopentadienylmagnesium (Cp₂Mg). TheCp₂Mg gas was intended for addition of magnesium to the mainsemiconductor layer 5 as a p-type conductivity determinant. The TMG gaswas introduced at a rate of 4.3 micromoles per minute, the NH₃ gas at arate of 53.6 micromoles per minute, and Cp₂Mg at a rate of 0.12micromoles per minute.

There was thus grown on the surface of the active layer 4 the mainsemiconductor layer 5 of p-type GaN. This main semiconductor layer 5 hada thickness of approximately 0.2 micrometer and an impurityconcentration of approximately 3×10¹⁸ cm⁻³.

With the LED chip fabricated by OMCVD as above, the component layers ofthe chip have proved to be all well aligned in crystal orientation. Thebuffer layer 2 grew in good conformity with the orientation to themonocrystalline silicon substrate 1, and the main semiconductor layers3–5 were well aligned in orientation with the buffer layer.

In order to complete the LED, the anode 6 _(a) was formed centrally onthe front surface of the LED chip by vacuum deposition of nickel andgold. The anode 6 _(a) made low-resistance contact with the surface ofthe p-type main semiconductor layer 5. Light is emitted from that partof the surface of the main semiconductor layer 5 which was leftuncovered by the anode 6 _(a). The cathode 6 _(b) was formed on theunderside of the chip by vacuum deposition of titanium and aluminum.

Constructed and fabricated as in the foregoing, the LED of FIGS. 1–3offers the following advantages:

-   1. The second complementary sublayers 10 of the multiple well active    layer 4, each interposed between one well sublayer 8 and one barrier    sublayer 7 thereover, prevent the evaporation of indium from the    underlying well sublayers during the deposition of the barrier    sublayers thereon. Thus the well sublayers 8 have their    crystallinity left unimpaired, and the barrier sublayers 7 are saved    from the deterioration of crystallinity by indium intrusion from the    well sublayers, assuring material improvement in the performance of    the LED.-   2. The first complementary sublayers 9, each interposed between one    well sublayer 8 and one barrier sublayer 7 thereunder, reduce the    mutual diffusions of gallium from the barrier sublayers and indium    from the well sublayers due to heat application during the    fabrication of the active layer 4 and main semiconductor layer 5 or    during any subsequent manufacturing processes.-   3. The second complementary sublayers 10 coact with the first 9    further to reduce the mutual diffusions of gallium and indium    between he barrier sublayers 7 and well sublayers 8, making these    sublayers still better in crystallinity and contributing toward    further enhancement of the LED performance.-   4. Indium evaporation from the well sublayers 8 is curtailed as the    second complementary sublayers 10 are made higher in aluminum    content, and greater in thickness, than the first 9.-   5. The first complementary sublayers 9 are prevented from becoming    too high in electrical resistivity as they are made less in aluminum    content and in thickness than the second complementary sublayers 10.-   6. The active layer 4 is kept from deterioration of crystallinity as    both complementary sublayers 9 and 10 restrict the diffusion of the    conductivity determinant of the main semiconductor layer 5 into the    active layer.-   7. Having a lattice constant intermediate those of the silicon    substrate 1 and GaN buffer sublayers 2 _(b), the AlN buffer    sublayers 2 _(a) faithfully conform to the crystal orientation of    the substrate. This in turn permits the GaN semiconductors of the    main semiconductor layers 3 and 5 and active layer 4 to be grown on    the buffer layer 2 with their crystal orientation aligned for high    intensity light emission.-   8. The main semiconductor layers 3 and 5 and active layer 4 are    formed with a high degree of flatness as they overlie a lamination    of a multiplicity of buffer sublayers 2 _(a) and 2 _(b). Should the    buffer layer consist solely of GaN semiconductor, the main    semiconductor layer 3 and other GaN layers would not be flat enough    by reason of too much difference in lattice constant between Si and    GaN. Also, the buffer layer would become too high in resistivity if    it consisted solely of a relatively thick AlN layer, and would not    provide a sufficient buffering action if it were a relatively thin    AlN layer. Made up of alternating first sublayers 2 _(a) of AlN,    which differs only slightly from silicon in lattice constant, and    second sublayers 2 _(b) of GaN, the buffer layer 2 according to the    invention makes it possible to form the GaN semiconductor layers    thereon with a high degree of flatness and crystallinity.-   9. The individual first sublayers 2 _(a) of the buffer layer 2 have    their thickness determined to offer the quantum-mechanical tunnel    effect, so that the buffer layer as a whole is held relatively low    in resistivity.-   10. The LED chip is kept from warping as a result of a difference in    thermal expansion coefficient between the silicon substrate 1 and    GaN semiconductor region such as the main semiconductor layer 3.    Being so different in thermal expansion coefficient, the silicon and    GaN layers when placed in direct contact with each other would    present a certain cause for warpage. This cause is precluded    according to the invention by the interposition of the laminated    buffer layer 2. The AlN sublayers 2 _(a) of this buffer layer are    intermediate in thermal expansion coefficient between silicon    substrate 1 and GaN main semiconductor layer 3, and the mean thermal    expansion coefficient of the complete buffer layer 2 is also    intermediate the thermal expansion coefficients of the substrate 1    and main semiconductor layer 3. Hence the elimination of chip    warpage.-   11. With the second buffer sublayers 2 _(b) made as thick as 30    nanometers, the appearance of discrete energy levels is minimized in    the valence band and conduction band of the second buffer sublayers,    resulting in prevention of an increase in energy level that has to    do with carrier conduction in these sublayers. That is to say that    both buffer sublayers 2 _(a) and 2 _(b) are saved from gaining a    superlattice state. As the discontinuity of the energy band between    substrate 1 and second buffer sublayers 2 _(b) is thus prevented    from worsening, the resistance between anode 6 _(a) and cathode 6    _(b) is reduced, and so is the voltage to be applied therebetween.

FIG. 4 shows an alternative form of LED embodying the principles of theinvention. This alternative LED represents a slight modification of itsFIG. 1 counterpart, being akin thereto except that the FIG. 4 device hasno n-type main semiconductor layer or lower confining layer 3. Theactive layer 4 directly overlies the n-type buffer layer 2 in themodified device, so that the buffer layer serves the purpose of a lowerconfining layer as well.

The modified LED gains the advantage that the active layer 4 is subjectto less tensile stress than in the FIG. 1 device in which the activelayer overlies the relatively thick lower confining layer 3. The activelayer 4 is therefore so much the better in crystallinity, contributingto the higher performance of the LED.

Notwithstanding the foregoing detailed disclosure it is not desired thatthe present invention be limited by the exact showings of the drawingsor the description thereof. The following is a brief list of possiblemodifications, alterations or adaptations of the invention which are allbelieved to fall within the purview of the claims which follow.

-   1. The substrate 1 need not necessarily be of monocrystalline    silicon but may for example be of polycrystalline silicon, a silicon    compound such as silicon carbide, or sapphire.-   2. The substrate 1, buffer layer 2, lower confining layer 3, active    layer 4 and upper confining layer 5 are all reversible in    conductivity types.-   3. The buffer layer 2 and confining layers 3 and 5 could be    fabricated from any such gallium- or indium-nitride-based compound    semiconductors as gallium nitride, aluminum indium nitride, aluminum    gallium nitride, indium gallium nitride, and aluminum indium gallium    nitride.-   4. The confining layers 3 and 5 could each be a lamination of    sublayers of different compositions.-   5. The anode 6 _(a) could be transparent.-   6. The first buffer sublayers 2 _(a) could be made greater in number    than the second buffer sublayers 2 _(b) by one, so as to occupy both    topmost and bottommost sublayers of the buffer layer 2.-   7. The second buffer sublayers 2 _(b) could likewise occupy both    topmost and bottommost sublayers of the buffer layer 2.-   8. The active layer 4 could be of a single-quantum-well, rather than    multiple-quantum-well, structure, in which case the active layer    might be constituted of one well sublayer 8, two barrier sublayers    7, and two complementary sublayers 9 and 10.-   9. Any one or more of the constitute sublayers 7–10 of the active    layer 4 could be doped with n- or p-type impurities.-   10. The cathode 6 _(b) could be coupled directly to the buffer layer    2.

1. A method of creating an active layer of quantum well structure on asemiconductor layer of a prescribed conductivity type in alight-emitting semiconductor device, the method comprising the steps of:(a) forming a barrier sublayer on the semiconductor layer byorganometallic chemical vapor deposition, the barrier sublayer being ofIn_(y)Ga_(1-y)N where the subscript y is a numeral that is greater thanzero and equal to or less than 0.5; (b) forming a first complementarysublayer on the barrier sublayer by organometallic chemical vapordeposition, the first complementary sublayer being ofAl_(a)Ga_(b)In_(1-a-b)N where the subscript a is a numeral that isgreater than zero; the subscript b is a numeral that is less than one;and the sum of a and b is equal to or less than one; (c) forming a wellsublayer on the first complementary sublayer by organometallic chemicalvapor deposition, the well sublayer being of In_(x)Ga_(1-x)N where thesubscript x is a numeral that is greater than zero, equal to or lessthan 0.5, and greater than y in the formula defining materials for thebarrier sublayer; (d) forming a second complementary sublayer on thewell sublayer by organometallic chemical vapor deposition, the secondcomplementary sublayer being of Al_(a′)Ga_(b′)In_(1-a′-b′)N where thesubscript a′ is a numeral that is greater than zero; the subscript b′ isa numeral that is less than one; and the sum of a′ and b′ is equal to orless than one; (e) forming another barrier sublayer on the secondcomplementary sublayer by organometallic chemical vapor deposition, thesecond recited barrier sublayer being of In_(y)Ga_(1-y)N where thesubscript y is a numeral that is greater than zero and equal to or lessthan 0.5.
 2. The method of claim 1 which further comprises cyclicallyrepeating, after step (e), steps (b), (c), (d) and (e) a preselectednumber of times for production of an active layer of multiple quantumwell structure.
 3. The method of claim 1 wherein the aluminum proportiona′ of the second complementary sublayer, or each second complementarysublayer, of the active layer is greater than the aluminum proportion aof the first complementary sublayer, or each complementary sublayer, ofthe active layer.
 4. The method of claim 1 wherein the secondcomplementary sublayer, or each second complementary sublayer, of theactive layer is thicker than the first complementary sublayer, or eachfirst complementary sublayer, of the active layer.
 5. A light-emittingsemiconductor device having an active layer of quantum well structurebetween two confining semiconductor layers of opposite conductivitytypes, the active layer comprising: (a) a barrier sublayer formed on oneof the confining semiconductor layers, the barrier sublayer being ofIn_(y)Ga_(1-y)N where the subscript y is a numeral that is greater thanzero and equal to or less than 0.5; (b) a first complementary sublayerformed on the barrier sublayer, the first complementary sublayer beingof Al_(a)Ga_(b)In_(1-a-b)N where the subscript a is a numeral that isgreater than zero; the subscript b is a numeral that is less than one;and the sum of a and b is equal to or less than one; (c) a well sublayerformed on the first complementary sublayer, the well sublayer being ofIn_(x)Ga_(1-x)N where the subscript x is a numeral that is greater thanzero, equal to or less than 0.5, and greater than y in the formula abovedefining materials for the barrier sublayer; (d) a second complementarysublayer formed on the well sublayer, the second complementary sublayerbeing of Al_(a′)Ga_(b′)n_(1-a′-b′)N where the subscript a′ is a numeralthat is greater than zero; the subscript b′ is a numeral that is lessthan one; and the sum of a′ and b′ is equal to or less than one; and (e)an additional barrier sublayer formed on the second complementarysublayer, the additional barrier sublayer being of In_(y)Ga_(1-y)N wherethe subscript y is a numeral that is greater than zero and equal to orless than 0.5.
 6. The light-emitting semiconductor device of claim 5further comprising a plurality of cyclic alternations of the firstcomplementary sublayer and the well sublayer and the secondcomplementary sublayer and the barrier sublayer, which are formed on theadditional barrier sublayer of the first recited set of such sublayersin order to provide a multiple quantum well active layer.
 7. Thelight-emitting semiconductor device of claim 5 wherein the aluminumproportion a′ of the second complementary sublayer, or each secondcomplementary sublayer, of the active layer is greater than the aluminumproportion a of the first complementary sublayer, or each complementarysublayer, of the active layer.
 8. The light-emitting semiconductordevice of claim 5 wherein the second complementary sublayer, or eachsecond complementary sublayer, of the active layer is thicker than thefirst complementary sublayer, or each first complementary sublayer, ofthe active layer.
 9. The light-emitting semiconductor device of claim 5wherein one of the confining semiconductor layers is a lamination ofalternating first and second sublayers, each first sublayer being ofAl_(x)Ga_(1-x)N where the subscript x is a numeral that is greater thanzero and equal to or less than one and having a thickness determined tooffer a quantum-mechanical tunnel effect, each second sublayer being ofAl_(y)Ga_(1-y)N where the subscript y is equal to or greater than zero,less than one, and less than x in the formula defining materials for thefirst sublayer, and having a thickness in the range of from about 10 toabout 500 nanometers, and being thicker than each first sublayer.